Multiscale Thermofluid Studies Shahram Pouya – Turbulent Mixing and Unsteady Aerodynamics Laboratory , Mechanical Engineering Develop novel optical measurement techniques to address unique challenges of fluid mechanics problem. Single Quantum Dot Velocimetry Molecular Tagging Velocimetry/Thermometry Application of femtosecond lasers Fluid Vorticity measurement using laser beams with OAM (Orbital Angular Momentum) Experimental study of various fundamental and applied fluid mechanics problems from micro to macro scales. Study of slip/no-slip boundary condition in microchannels. Surface roughness and slug flow in microreactors. Diffusion of nano-particles in confined geometries. Characterization of flow in NASA ZBOT experiment. Molecular Tagging in air Flow disturbance induced by femtosecond laser pulses. Collaborations Prof. Dantus. Prof. Blanchard (Chemistry) Prof. Koochesfahani, Prof. Naguib, Dr. Yeom (Mechanical Eng.) Single QD Velocimetry MTV with femtosecond excitation Vorticity measurement using OAM beam   NASA ZBOT Slug flow in microreactors

Single Quantum Dot (QD) Velocimetry Using Evanescent field illumination to excite and image single (6 nm core size) Quantum Dots within 100-300 nm of the surface. Individual QDs are tracked in time to generate an ensemble of displacement/velocity data. Extracting statistical quantities is not trivial Exposure effect Bias diffusion near wall Using correction factors based on Brownian diffusion simulation, a difference in slip length of 31 nm was measured between hydrophilic and hydrophobic channels. ICCD camera Quartz microchannel (200 μm gap) Syringe pump 0.5 mL/min ( =5.2 mm/s, Re=202) θ > θc Evanescent field More info: Pouya et al., EIF 2005 - Pouya et al. APS-DFD 2006 - Pouya et al., JFM 2008

Vorticity measurement using OAM beams Vorticity is a fundamental parameter in fluid mechanics yet no direct measurement of it is available. Direct non-intrusive measurement of vorticity. Relies on Rotational Doppler Effect to measure the angular velocity of a rotating object (similar to LDV used for measuring translational velocity). An OAM beam imposes a spatial modulation of the phase front. Successful measurements were demonstrated for a simple flow (solid body rotation) using single 100 micron particles. Detector DOF ~ 3 mm Dichroic mirror 488nm Laser beam 55 mm Time series and power spectrum of collected signal for two different rotation frequencies of the cylindrical container Ryabtsev et al., Optics Express 2016

Velocimetry and Thermometry in NASA ZBOT ZBOT is a microgravity experiment being developed for operation on the ISS (as a model for cryogenic storage for Mars missions). It examines an innovative ventless means of controlling tank pressure through active heat removal and forced liquid jet mixing. It uses a simulant-fluid (perfluoropentane or PnP) with low boiling temperature. There has been issues with using particle based techniques (PIV), hence the interest for molecular based methods. We are developing molecular based methods that can be used for this experiment Very few molecular tracers into PnP! Identified Coumarin 153 as a dye that dissolves. Using expertise of Prof. Blanchard’s lab identified fluorous group that help C153 going into PnP Synthesized QDs with similar group attachments to make them soluble ! Modified QDs have been shown to have good temperature sensitivity Velocimetry based on MTV with photobleaching appear possible. T=23 °C Emission intensity of QDs is temperature dependent T=47 °C Laser sheet used for interrogation of tagged region Laser beam used for molecular tagging by photobleaching Thermocouple

MTV/MTT using Femtosecond excitation Using femtosecond pulse of ultrafast infrared lasers for multi-photon resonant excitation of the water-soluble phosphorescent triplex for MTV/MTT measurement. Multi-photon excitation alleviates one of the constraints in the use of MTV/MTT, the need for short wavelength UV lasers to create the molecular tags and UV optical access in flow facilities. It also offers the opportunity for MTV imaging at high repetition rates.   Ambient Co-Flow T~ 26 °C Cold Jet Flow ~ 15 °C @ exit Re ~ 400 Femtosecond Laser Beam @ 400 nm d=2 mm ID 50 mm 2.2 mm 3.8 mm 140µs after laser pulse 4.2 ms delay after first image